RUSKIN, Florida (CNN) -- The six-day search for 13-year-old Sarah Michelle Lunde ended Saturday when searchers found her body partially submerged in a pond near her Florida home.

Hillsborough Sheriff David Gee said the pond at an abandoned fish farm will be drained Sunday and the surrounding area searched for possible evidence.

Sarah's family discovered her missing early last Sunday.

Lunde was wearing a green cast when she disappeared, and Gee told reporters, "the green cast is apparent" on the body.

"The family is very distraught, obviously. It's a tough time for them, and we're trying to assist them in dealing with this," Gee said.

He refused to comment on the condition of the body, but said it appeared to have in the pond several days. It's likely Sarah was killed soon after her disappearance, he said.

Area residents were asked to turn over any clothing that might belong to Sarah that they find near where the body was found.

"We will do anything in our power to bring the person or persons to justice," Gee said. "I believe we eventually will find enough evidence to charge the person."

A search-and-rescue dog found the body half-submerged at an abandoned fish farm a half-mile south of the girl's home, the sheriff said. It was in an area that had been searched before.

"It was clear from investigators that whoever put her there went to great effort to conceal her body," Gee said.

The sheriff said he told Sarah's mother, Kelly May, this morning about the discovery.

"She has family around her and some friends, and it's a very difficult time," Gee said earlier. "They're obviously very upset. They're very worried at this news."

Beer bottles

Since Sarah's disappearance, authorities have been questioning David Onstott, a 36-year-old convicted sex offender -- currently in custody on unrelated charges -- who recently ended a relationship with the missing girl's mother.

Police have not called Onstott a suspect, and his attorney denies that Onstott had anything to do with Sarah's disappearance.

On Friday, searchers were asked to keep an eye out for empty Bud or Bud Light beer bottles in the vicinity that may have a connection to Onstott.

According to Sarah's 17-year-old brother, Onstott was at the family home early Sunday morning -- hours after she was last seen. The brother told police that it was the first time he had seen Onstott in months, and that Onstott picked up a half-full beer bottle from a table before leaving.

Sarah was last seen by her brother at her home when she returned from a church function April 9. Her brother went out, and when he returned early Sunday she was gone. Her mother was out of town and reported her missing Monday. A national missing-persons alert was issued Tuesday.

Troubled histories

Authorities arrested Onstott on Tuesday in Apollo Beach, Florida, north of Ruskin, on a charge of aggravated assault with a deadly weapon, the sheriff's office said. Deputies said Onstott threatened a man with a screwdriver during an argument.

He was arraigned Thursday on a charge of failing to register as a sex offender, and remains in the Hillsborough County Jail. Although his bond was set at $250,000, Onstott also was being held on a drunken driving warrant from Michigan, sheriff's spokesman J.D. Callaway said. Because there is no bond on the Michigan charge, Onstott is being held without bond, Callaway said.

Onstott has a 1995 sexual battery conviction for assaulting a female acquaintance in her home, for which he served six years in prison and two years probation, Callaway said.